welding-s

Welding Practice

Joint Preparation and Cutting

Cutting or preparation of edges should be carried out by guillotine, machining, grinding or plasma arc. Edges cut by plasma arc should be smooth and free from gutters or notches and shall have oxides removed. All other edges should be deburred. Any cutting or preparation carried out with the carbon arc process or by powder cutting should have 1.5mm dressed from the cut edge. Carbon arc gouging is not recommended for cutting of stainless steels under any circumstances. All spatter is to be removed and the surface of the parent metal dressed smooth.

 

Electrode Care

Electrodes that have been removed from their packets should be transferred to a holding oven or welder’s hot box and maintained at a temperature of 110ºC to prevent moisture pick-up until required for use. Electrodes that have become damp should be redried at 250ºC for 2 hours prior to use.

 

General Welding Practice and Technique

Cleaning

Cleaning may be necessary before welding and during welding (interpass) and is usually essential after welding in order to ensure maximum corrosion resistance.

Pre-weld cleaning involves dressing the cut edge and removing all contaminants such as oil, paint, grease, crayon marks, adhesive tapes, etc. The area on both sides of weld should be cleaned before welding by brushing with a clean stainless steel brush and wiped with a solvent moistened cloth. All moisture must be removed and if a flame is used care must be taken to see that any water (a product of combustion) does not remain on the surface or in the weld preparation. Liquid petroleum gas particularly creates a large amount of water when burnt.

Each welding run must be thoroughly cleaned to remove slag and spatter before proceeding with the next run. The cleaning method used (chipping, brushing, grinding) will depend on the welding process, bead shape, etc. but care should be taken to see that the weld area is not contaminated in the process. Any cleaning equipment should be suitable for stainless steel and kept for that purpose.

During welding, a gas purge on the reverse side may be advantageous.

After welding, weld spatter, flux, scale, arc strikes and the overall heat discolouration should be removed. This can involve grinding and polishing, blasting and brushing with a stainless steel wire brush, or use of a descaling solution or paste. The preferred procedure is usually dictated by end use.

Grinding and dressing is to be carried out with iron-free brushes,abrasives, etc. and should not be so heavy as to discolour and overheat the metal. Rubber and resin bonded wheels are satisfactory. Wheels should be dressed regularly to prevent them becoming loaded thereby producing objectionable scratches. In any blasting process steel shot shall not be used.

 

Welding Procedure

Efficient arc striking is essential in welding all types of stainless steels as indiscriminate arc striking tends to scar or burn the surface of the steel thus providing areas for premature chemical attack. Care and skill are required to obtain satisfactory weld starts and restarts on continuous seams. As both burn-through on light section material and cold lack of penetration starts on heavier material can easily result from imperfect technique, the suggested procedure is to strike the electrode in the joint approximately 8-10mm forward of the actual start point. With the arc established and electrode correctly angled it can then be rapidly taken to the start point for complete fusion of the previous weld and/or the joint root of a new weld.

Efficient tack welding on austenitic steels is essential in controlling distortion. The length of tack welds varies from 12-40mm depending on sheet or plate thickness. Generally tack welds are more closely spaced than when welding mild steel of similar dimensions.

The use of a long arc or excessive current is capable of causing losses of manganese and chromium which can impair the corrosion resistance of the weld joint. Arc length should therefore be kept as short as possible consistent with satisfactory welding performance, ie. minimum spatter, complete fusion, acceptable bead shape.

Welding should be carried out at the lowest current consistent with good fusion at the selected welding speed to minimise heat input and control distortion. Higher welding speeds can be an advantage.

Stringer beads are recommended in preference to weaving in order to keep the heat input to an acceptable level. Where weaving is necessary both the weave width and side dwell time should be kept to a minimum. Interpass temperatures generally should not exceed 150ºC except in the case of martensitic alloys.

Breaking the arc in an abrupt manner can result in slag inclusions and shrinkage cracks. Craters should be filled by using a circular motion at the end of the weld followed by a gradual lengthening of the arc to the point of extinguishing it.

Welding conditions and welding technique should be such as to produce a smooth weld surface which requires minimum dressing.

Fillet weld beads shall be of full throat thickness and correct contour, consistent with good operation. Concave fillet welds are to be avoided.

The root side of the weld must be protected against oxidation especially in gas-shielded arc welding. Protection with shielding gas is commonly applied. Back-gouging (grinding) of the root and welding from the reverse side of the joint can also be used when the design so permits.

 

Recommended filler metals for dissimilar metal joint welding

 

Parent 201 303(1) 309 310 317 317L 321 S30815
Metal 202304309S310S316316L347 (253MA)
ASTM  304L   316Ti  
(AISI)        
201 308 308 308 308 308 308 347 308
202 308L 308L 308L 308L 308L 308L 318 347
    312 347 347 347 318 308  
          318      
304(1)   308 308 308 308 308 347 22.12.HT
304L   308L 308L 308L 308L 308L 308 308
303   347 347 347 347 347 308L 347
              318  
309     309 309 309 309 374 22.12.HT
309S     309L 309L 309L 309L 308 309
        310   316L 308L 347
            318    
310       310 317L 317L 347 22.12.HT
310S       310L 316L 316L 308 310
          318 318 308L 309
          309 309 310  
            309L    
317         317 317L 347 22.12.HT
316         316 316L 318 309
          318 316 316  
            318    
317L           317L 347 22.12.HT
316L           316L 318 309
316Ti           318 308  
              316L  
321             347 22.12.HT
347             318 309
              308 347
S30815               22.12.HT
(253MA)                
                 
409                
410                
430                
446                
                 
                 
S31500                
S31803                
S32304                
NiCrFe(2)                
Alloys                
                 

 

Parent 409 446 Duplex NiCrFe(2)
Metal410  S31500 Steels
ASTM430  S31803 
(AISI)   S32304 
201 309 309 22.8.3L NiCr-3
202 310 310 309 NiCrFe-6
         
         
304(1) 309 309 22.8.3L NiCr-3
304L 310 310 309L NiCrFe-6
303     309  
         
309 309 309 22.8.3L NiCr-3
309S 310 310 309 NiCrFe-6
      309L  
         
310 309 310 22.8.3L NiCr-3
310S 310 309 309 NiCrFe-6
      309L  
         
         
317 309 309 22.8.3L NiCr-3
316 310 310 309Mo NiCrFe-6
      309  
         
317L 309 309 22.8.3L NiCr-3
316L 310 310 309Mo NiCrFe-6
316Ti     317L  
      316L  
321 309 309 22.8.3L NiCr-3
347 310 310 309 NiCrFe-6
         
S30815 22.12.HT 22.12.HT 22.8.3L NiCr-3
(253MA) 309 309 309 NiCrFe-6
  310 310 310  
409 410 446 22.8.3L NiCr-3
410 309 310 309 NiCrFe-6
430   309 309L  
446   446 309 NiCr-3
    310 309L NiCrFe-6
    309    
S31500     22.8.3L NiCr-3
S31803     309Mo NiCrFe-6
S32304        
NiCrFe(2)       NiCr-3
Alloys       NiCrFe
        Alloys

 

Parent Carbon(1) Low(1) 501
Metal Steels Alloy Steels 502
ASTM  505
(AISI)   
201 309 309 309
202      
       
       
304(1) 309 309 309
304L      
303      
       
309 309 309 309
309S      
       
       
310 310 310 310
310S 309 309 309
       
       
       
317 309 309 309
316      
       
       
317L 309 309 309
316L      
316Ti      
       
321 309 309 309
347      
       
S30815 22.12.HT 22.12.HT 22.12.HT
(253MA) 309   309
  310   310
409 309 309 309
410      
430      
446 309 309 309
       
       
S31500      
S31803      
S32304      
NiCrFe(2)      
Alloys      
       

 

Notes:
(1) This group includes free-cutting steels. When such a steel is a member of the joint certain precautions have to be taken. Buttering the free-cutting steel with 312 before welding the joint with a filler metal that suits the other part of the joint or welding the whole joint with 312 is normally a safe procedure.

(2) Higher strength can be obtained by using NiCrFe-6 with subsequent heat treatment.

General Notes:
• If the dilution is high, eg. in submerged arc welding, special high ferrite grades are often preferred.

• If the working conditions require heat treatment, the filler metal choice may have to be reconsidered. Owing to the infinite combinations of materials and working conditions, no general rules can be applied.

• Filler metals are stated in order of preference. Normally, MMA, TIG, sub-arc welding is assumed. For MIG welding grades with higher silicon contents, eg. 308LSi, 308Si are preferred.

• Where 309 is specified 309MoL may be used. Where 309 is specified filler metals 310, 312, NiCr-3 may generally be used, however, care must be exercised with this selection: eg. i) to avoid high ferrite levels (312 consumable) which may lead to sigma phase embrittlement, ii) to avoid high nickel contents (NiCr-3) which can be attacked in sulphur bearing high temperature environments.

• Where designated consumables are not available, more highly alloyed grades may be used. However, due care must be taken with their selection.

• For high temperature transition joints carbon diffusion has tobe considered. In such cases, 310 or NiCr-3 is recommended.

• When joining dissimilar but highly corrosion resistant steels, ferrite-free deposits are often demanded. Although each case has to be considered separately, the use of NiCr-3, 20.25.5LCu, or 27.31.4LCu can often be recommended.

• In addition to the grades specified here filler metals with specific properties are available, eg. low ferrite content, high carbon, extra low interstitials, high purity, etc.

• This table is not exhaustive. Other alloys may also be suitable.

 

Passivation

Passivation is the treatment of the surface of stainless steels, often with acid solutions (or pastes), to remove contaminants and promote the formation of the passive film on a freshly created surface (eg through grinding, machining or mechanical damage).

Common passivation treatments include nitric acid (HNO3) solutions or pastes which will clean the steel surface of free iron contaminants. Care must be taken in selecting and using passivation treatments to ensure the selected treatment will target the contaminant. Passivation will also aid in the rapid development of the passive oxide film on the steel's surface. Passivation does not usually result in a marked change in appearance of the steel surface.

Both pickling and passivation solutions can employ dangerous acids that can damage both the operator and the environment if not handled correctly. Stainless pickling acids are highly corrosive to carbon steel.

It is essential that all acids are thoroughly removed by rinsing the component after completing the process. Residual hydrofluoric acid will initiate pitting corrosion.

It may be advantageous to neutralise the acid with an alkali before the rinsing step.

ASTM A380 Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems is a valuable source of information on pickling and passivation treatments. Other sources of information may be obtained by contacting ASSDA.

The corrosion resistance of the stainless steel is affected by the roughness of the surface after polishing, with a marked decrease of the corrosion resistance as the surface roughness increases above a Ra value of about 0.5 micrometres. This roughly corresponds to the surface produced by grinding with 320 grit abrasives.

Either passivation or electropolishing can be used to improve the corrosion resistance of mechanically polished surfaces.